1. Field of the Invention
The present invention relates to an intruder identifying method, an intruder identifying device and an intruder identifying sensor device for disposing a leakage coaxial cable in an area or at a boundary where intrusion of an intruder is required to be monitored, and identifying, with electric waves, an intruder as a detection target which intrudes into an intrusion preventing area by approaching, traversing or overpassing the leakage coaxial cable.
2. Description of the Related Art
According to a conventional intruder detecting device for disposing a leakage coaxial cable in an area or at a boundary where intrusion of an intruder is required to be monitored, and detecting an intruder which traverses the leakage coaxial cable and intrudes into an intrusion preventing area, a reception signal comprising electric waves is transmitted from a transmission leakage coaxial cable, and it is received by a reception leakage coaxial cable to monitor the level of the reception signal. When an intruder intrudes between the transmission and reception leakage coaxial cables, the electric waves are interrupted, and thus the level of the reception signal is lowered. The level of the reception signal is determined on the basis of a predetermined threshold value at all times. Therefore, when the level of the reception signal is lower than the threshold value, “intrusion of intruder” is determined, and an alarm is output (for example, see Japanese Patent No. 3,703,689 (FIG. 1 and the description thereof), JP-A-10-95338 (FIGS. 3 and 4 and the descriptions thereof).
The conventional intruder detecting device using the leakage coaxial cable merely determine the reduction of the level of the reception signal on the basis of the threshold value. Accordingly, the reception level of the reception signal received through the reception leakage coaxial cable is also lowered in such a case that an object which is not originally required to be detected, for example, a small flying object such as a bird or the like, a small animal such as a dog, a cat or the like, water drops falling from leaves of trees in cluster due to wind or the like when the leakage coaxial cable is laid in woods, comes into contact with the leakage coaxial cable.
In the case of even a small animal, the variation amount of the reception signal is greater as it is nearer to the leakage coaxial cable, and thus there occurs such a problem that the level of the reception signal is lowered and an erroneous report “intruder intrudes” is output.
Furthermore, there is also a problem that the leakage coaxial cable itself or the ground surface, a walls or a pole around the leakage coaxial cable gets wet with falling rain, and thus the reflection and transmission coefficients thereof vary, so that the level of the reception signal is varied and thus an erroneous report “intruder intrudes” is output.
Still furthermore, the conventional intruder detecting device outputs an alarm “intruder intrudes” instantaneously when the level of the reception signal underruns the threshold value. Therefore, the level of the reception signal is lowered at the instantaneous time when a small flying object, a small animal or water drop which is not originally required to be detected comes into contact with the leakage coaxial cable as an antenna, and the threshold-value determination is made under this state, so that an erroneous report “intruder intrudes” is output.